Clavulanic acid, (2R,5R,Z)-3-(2-hydroxyethylidene)-7-oxo-4-oxa-1-azabicyclo-[3,2,0]-heptane -2-carboxylic acid, is a known compound of the following structure: ##STR3##
This compound as well as the salts and esters thereof are active as inhibitors of betalactamase, i.e. they inhibit beta-lactamases produced by gram-positive and gram-negative microorganisms. Hence clavulanic acid and its salts are used in gelenic preparations in order to inhibit the inactivation of beta-lactam antibiotics. Commercial preparations contain a stable potassium salt of clavulanic acid (clavulanic acid itself being rather unstable) in combination with amoxicillin trihydrate.
Clavulanic acid is obtained by a fermentation method from various microorganisms belonging to various Streptomycetes strains such as S. clavuligerus NRRL 3585, S. jumoninensis NRRL 5741, S. katsurahamanus IFO 13716 and Streptomyces sp. P 6621 FERM P2804.
Clavulanic acid and its salts were first disclosed in GB 1,508,977. However, the process for the preparation of clavulanic acid described therein is time-consuming and is based on exacting purifications by means of various chromatographic methods. The salts of clavulanic acid are obtained by binding the clavulanate anion present in the filtrate of the fermentation broth on an anionic exchange resin, subsequent elution of the clavulanate anion therefrom by means of an electrolyte, desalting the obtained eluate, passing the latter through another anionic exchange resin and a subsequent chromatographic elution therefrom by means of an electrolyte, repeatedly desalting the obtained eluate and removing the solvent.
The process requires the use of preparative chromatographic columns, which represents a considerable investment expenditure; in addition, the applicability on a large scale is limited.
A further drawback of this process is the fact that the most steps thereof are carried out in an aqueous medium where clavulanic acid is very unstable.
GB 1,543,563 discloses a modified fermentative process, wherein the pH value of the medium is maintained within the range from 6.3 to 6.7, which results in an increased yield of the desired compound. The salts of clavulanic acid such as potassium clavulanate are obtained from lithium clavulanate by double exchange.
According to the process for the preparation of clavulanic acid and of pharmaceutically acceptable salts thereof as disclosed in EP-0-026044, the exacting methods of purification by means of exchange resins are largely avoided. The process is based on the preparation of the tert. butylamine salt of clavulanic acid, preferably in the form of its acetone solvate. The tert. butylamine salt of clavulanic acid is prepared by treating the extract, preferably the ethyl acetate extract, containing crude clavulanic acid, which was prepared according to the process as described in GB 1,508,977, with tert.butylamine in an organic solvent such as acetone, followed by the conversion of the isolated tert. butylamine salt of clavulanic acid to clavulanic acid or a pharmaceutically acceptable salt thereof.